The present invention relates to high pressure feeders (HPF) typically used to pressurize slurries of comminuted cellulosic material, such as wood chips. The present invention particularly relates to screens that retain cellulosic material in the rotor of the HPF and allow liquid to be discharged through a low pressure outlet of the HPF.
High pressure feeders are typically included in chip feed systems that deliver a high pressure slurry of wood chips to a pressurized digester vessel, such as used in Kraft pulping. HPFs are described in U.S. Pat. Nos. 4,107,843; 6,120,646; 5,236,285, 6,468,006 and 6,616,384, and in published international patent applications WO 94/21855 and WO 99/42653.
HPFs typically have a rotor with four through passages that fill and empty with cellulosic material and liquid as the rotor turns in a housing. HPFs typically have a low pressure outlet through which passes liquor separated from the slurry of cellulosic material.
To prevent the loss of cellulosic material, a screen grid typically covers the low pressure outlet. The screen grid passes low pressure liquor through to the low pressure outlet and blocks cellulosic material from exiting through the low pressure outlet. In conventional HPFs, the screen grid has a narrow solid section in the center that is aligned with a divider bar in the casing of the HPF. The center-section of the screen grid abuts the outlet divider bar in the middle of the low pressure outlet of the HPF housing and below the screen grid. The divider bar is a narrow support wall extending across the center of the low pressure outlet.
The divider bar and narrow solid section of the screen grid are generally aligned with a center solid circumferential portion of the rotor of the HPF. This alignment minimizes the flow obstruction caused by the divider bar and narrow solid section. The alignment between the outlets in the rotor and the screen grid and the divider wall changes as the rotor is moved a relatively short distance, e.g., 5 inches (127 millimeters (mm), axially with respect to the liner and casing of the HPF. The rotor is periodically moved to compensate for wear on the surfaces of the rotor and the liner. Moving the rotor axially changes the alignment between the outlets in the rotor and the center solid portion of the screen grid and the divider wall. As the alignment changes, the center solid portion of the screen grid and the divider wall may become misaligned with the center of the rotor and thereby obstruct the flow of liquor passing through the low pressure outlet of the HPF.
Conventional HPFs have dual screen grids with slots that do not extend continuously over the entire area of the screen grid exposed to the ports in the rotor and the low pressure outlet in the casing. Because the region of slots is not continuous, the screen grid has solid sections that obstruct the low pressure flow of liquor from the HPF. The rotor is obstructed specifically near the outlet divider bar and the drive end of the housing. The divider bar and narrow solid section of the screen grid obstruct the flow of liquor through the outlets in the rotor and the low pressure outlet in the casing of the HPF. The obstruction of the flow of liquor to the low pressure outlet reduces the efficiency and capacity of the HPF.